The invention relates to circuits for controlling the phase of a sinewave, and particularly to a simple, temperature stable, voltage controlled phase shifter for remotely shifting the phase of an output signal relative to an input signal.
In the television field, wherein several hand-held and/or studio cameras are generating encoded pictures, it is necessary to control the phase of the subcarrier signal that is fed to the encoder. That is, each camera must not only be genlocked together, but must also be phased with respect to all other cameras in the studio. A phased condition is required, for example, whereby camera output signals fed to a switcher may be properly cut, mixed, faded, etc., without loss of color information.
Several separate procedures are employed to genlock cameras together. First, the camera's vertical system is locked to a vertical reference. Second, the horizontal system is locked to a horizontal reference. Third, the camera's subcarrier is locked to a subcarrier reference. However, although the frequencies are matched, there is no assurance that the cameras are phased, i.e., there may be phase differences due to different lengths of cable, etc. To date, phase adjustments for correcting any phase differences have been made by opening the side of a camera and manually making phase adjustment by means of a screwdriver. Such manual adjustments are particularly undesirable in the small, hand-held, high performance cameras of recent development, wherein the encoders are built into the camera head. Further, in a fully automated camera such as typified by the above cross-referenced applications, assigned to the same assignee as this application, such a scheme is cumbersome, time consuming and generally undesirable.
Remote controlled phase shifting is available in one example of the prior art, wherein a relatively complex two channel system is employed, first to make a coarse phase correction, and thereafter a fine phase correction. In the case of a portable camera, where space, size and power consumption are of utmost importance, such a complex system is correspondingly less desirable. Such a system is illustrated by way of example only, in U.S. Pat. No. 4,183,044 to D. W. Breithaupt.
The basic problem in the design of a subcarrier phase shifter is that of stability. Thus there is available a sinewave of an amplitude of generally two volts peak-to-peak. The phase shifting circuit should have the capability of varying the output signal with respect to the input signal over a range of at least 360.degree.. However, in matching the two signals, the subcarrier phases must be detected and shifted within a degree or so of each other. One degree at the subcarrier frequency corresponds to a time increment of the order of two-thirds of a nanosecond in the NTSC color television standard, and even less time in the PAL standard. Therefore, it follows that the circuit must be as simple as possible and must be extremely stable with respect to temperature, drift, etc., while still providing the full range of 360.degree. of control.